test(paste): pin the main pane's keydown hand-off, and say which engines still get two pastes (#799)

The two edits promised in the review of #799:

- The main pane's Ctrl+V branch hands the keydown to the paste trap, and
  nothing tested that call: with it reverted to `_handleImagePaste()` every
  test still passed and Firefox would paste twice again. A source pin now
  fails on that, like the Ctrl+C one in terminal-copy-clean.test.ts.
- architecture-invariants and src/web/CLAUDE.md described the key's own paste
  as reaching the trap in every browser. Since #799 that is Chromium and
  WebKit only: in Firefox the trap cancels the keydown.

Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>

docs/architecture-invariants.md

@@ -956,9 +956,9 @@ Feedback is silent on success except ONCE per page load (a feature that works by
 **The Ctrl+V paste trap consumes exactly ONE paste event.** `_handleImagePaste()` (image-input.js) appends a hidden `contenteditable` div, focuses it, and reads the clipboard out of the paste event that lands there. An image becomes an upload whose saved path is typed into the session; plain text is a Codeman-built bracketed paste (`\x1b[200~` … `\x1b[201~`, nested markers stripped, newlines as `\r`) handed to that pane's terminal via `terminal.input(framed)`. `terminal.paste()` only emits those markers while xterm's DECSET 2004 mirror is on, and a terminal replay clears that mirror (#716), so Ctrl+V builds the frame itself. `input()` fires onData as user input, which scrolls to the bottom and clears the selection. The main pane's onData bracketed-paste branch and a tile's `_onTerminalData` deliver the frame, which is where the local-echo flush and the delayed write live (#219). A frame wrapped in CSI 200~/201~ does not match the query-reply or mouse-report filters. The terminal is refocused; the paste is not written into the xterm buffer. Two independent routes deliver that event for one keypress, and a browser may fire both:
 
 1. **`document.execCommand('paste')`**, which the function calls itself. ⚠️ Its return value proves nothing about whether it fired. Firefox dispatches a trusted `paste` event carrying the real clipboard and still returns `false`, because the trap cancels the event and the command therefore never completes. Chromium refuses the command outright and dispatches nothing.
-2. **The keydown's own default action.** The `Ctrl+V` branch in `attachCustomKeyEventHandler` returns `false`, which stops xterm from evaluating the key into `^V` but does not cancel the DOM event, for the same reason rule 1 of smart copy above spells out. `trap.focus()` has already run by then, so the browser sends its own paste to the trap as well.
+2. **The keydown's own default action.** The `Ctrl+V` branch in `attachCustomKeyEventHandler` returns `false`, which stops xterm from evaluating the key into `^V` but does not cancel the DOM event, for the same reason rule 1 of smart copy above spells out. `trap.focus()` has already run by then, so the browser sends its own paste to the trap as well. That holds for Chromium and WebKit, where it is the only paste. In Firefox the trap cancels the keydown once route 1 has delivered (below), so route 2 no longer happens there.
 
-Measured against a live install, one `Ctrl+V` each: Firefox delivers two paste events, Chromium and WebKit one. Handling both wrote the clipboard text to the PTY twice, which is why `Ctrl+V` pasted twice while right-click → Paste pasted once. That menu path involves no keydown, so route 2 cannot exist for it.
+Measured against a live install before the keydown was cancelled, one `Ctrl+V` each: Firefox delivered two paste events, Chromium and WebKit one. Handling both wrote the clipboard text to the PTY twice, which is why `Ctrl+V` pasted twice while right-click → Paste pasted once. That menu path involves no keydown, so route 2 cannot exist for it.
 
 ⚠️ **Once route 1 has delivered, the trap cancels the keydown.** Firefox dispatches the `execCommand` paste synchronously, inside the call. It delivers the key's own paste as a separate task about 10 ms later, measured in Playwright's Firefox. That later event races the `setTimeout(0)` that removes the trap and refocuses the terminal. When the timer wins, the event lands on xterm's textarea, and xterm's own `handlePasteEvent` sends the clipboard a second time; the trap's one-shot flag never sees that event. This race is why a one-shot flag alone left `Ctrl+V` pasting twice "sometimes". Both callers, the main pane's key handler and a tile's, pass the keydown as `target.keyEvent`. `_handleImagePaste()` calls its `preventDefault()` when the paste listener has already run by the time `execCommand` returns, so the browser never issues route 2. Chromium and WebKit dispatch nothing from route 1, so they keep the default action, which delivers their only paste event. ⚠️ Never cancel the keydown unconditionally: in those engines that loses the paste entirely.